- Excuse me, is this seat taken?
- Yes, this one is taken, but that seat on the left is vacant.

Another short-form adjective должен (должна, должно, должны)
is used to express necessity. It means "must" or "have to" and always followed
by a verb infinitive.

Он должен работать.

He has to work.

Notes

Short-form adjectives agree with the grammatical subject of a sentence in
gender and number. Masculine ending for a short-form adjective is -Ø
(zero), feminine ending is -а, neuter ending is -о,
and plural ending is -ы. Listen and compare the forms. Note how strress
changes in занят, занята, занято, заняты and in должен,
должна, должно, должны.

1. Антон занят. Он должен работать.

Anton is busy. He has to work. (subject is masculine)

2. Таня занята. Она должна работать.

Tanya is busy. She has to work. (subject is feminine)

3. Это место занято, а то свободно.

This seat is taken, but that one is vacant. (subject is neuter)

4. Студенты заняты. Они должны заниматься.

Students are busy. They have to study. (subject is plural)

The pronoun вы is plural, even if the subject addressed is only one
person (formal polite Вы). Compare: